The process of fabricating photomasks (usually chrome or iron oxide on soda, lime, glass or quartz) typically leaves behind several defects. The defects can be classified as opaque or clear; the former relating to the case where excess chrome exists in an unwanted area, the latter relating to the case where chrome is missing from a desired area. These defects can also arise during use of the mask in standard lithographic processes.
A photomask used in integrated circuit production is a flat optical glass plate having an opaque pattern defined by a thin film of deposited metal, commonly chrome. A clear defect in such a mask is an area of glass that should have an opaque metal film but does not, and hence is clear. An opaque defect in a mask is an area of film on the glass where it should not be. Repairing mask defects has become more difficult as the scale to which masks are made has become smaller. Modern masks are made to definitions of 1 to 2 microns.
Currently, it is possible to repair opaque defects (i.e., remove excess chrome) by laser vaporization. The repair of clear defects is much more complex.
The conventional method of repairing clear defects relies on a lift-off process, i.e. spinning on photoresist, sputtering on a metal, followed by liftoff. This process is not only time consuming and expensive, but also runs the risk of introducing new defects since the entire photomask is affected. In addition, high resolution, e.g. the repair of a 2 m diameter pinhole, may be difficult to achieve.
Another more site-specific method exists. Here, a microdrop of ink is dispensed on the clear defect, and then the entire photomask is baked in an oven to promote adhesion. This process is of limited resolution. Defects less than 10 m square usually cannot be repaired. Furthermore, the repair is not durable--it is easily removed during standard photomask cleaning procedures.
Another site-specific method for defect repair employs an ultraviolet laser (typically at 257 nm). Here, the laser is used in conjunction with a metal-bearing gas. The laser light photo-dissociates (i.e., breaks the molecular bonds) the molecules in the vapor phase. The metal fragments subsequently impinge on the surface eventually forming a thin film.
This procedure is difficult to employ in the semiconductor industry. Ultraviolet lasers generally occupy an inordinate amount of space. Since clean room space, where the repair takes place, is limited and costly, such a repair system is undesirable. In addition, ultraviolet laser light particularly that generated at 257 nm, is unstable and unreliable in a production environment. The required ultraviolet optics is also difficult to obtain and costly.
In terms of laser-based systems for opaque defect repair, several problems are also encountered. For example, such systems depend on mechanical scanning stages for the positioning of the defect area under the laser irradiation zone. to avoid the large costs involved, manufacturers typically employ low-resolution stages--typically 10 .mu.m for positioning. Finer adjustments must be made manually. This slows down the repair process and makes the accurate repair of small defects on the order of 1 m very difficult.
Further, such systems do not have the capability of scanning over small areas say 10 .mu.m square. Thus, to remove excess chrome in such an area would require several individual laser repairs, i.e. the area in which chrome is to be removed would have to be brought under the laser irradiation zone in several discrete steps.
Apparatus is commercially available for repairing opaque mask defects by focusing laser light energy on the defect to vaporize and scatter the film molecules. Such apparatus using microscope optics has become quite automated, including computer control of the optical X-Y table and the laser source so that a mask can be scanned, the position of opaque defects noted and stored as compared to a proper mask pattern, and then placed in the apparatus for computer controlled positioning of the mask and operation of the laser to ablate the defects. By tightly focusing the laser through the optics of such an apparatus, high definition in correction is achieved.
To the present time, such apparatus cannot, with the same precision and control, correct clear defects in photomasks. However, a method has been developed offering precision and control in the use of a laser for curing clear defects--see U.S. Pat. No. 4,543,270 which is hereby incorporated by reference.